Relative Stereochemical Determination and Synthesis of the C17–C25 δ-Lactone Fragment of Hemicalide

Hemicalide is a novel marine metabolite polyketide distinguished by a unique mechanism of action. Because of insufficient quantities of purified material, this natural product has evaded complete stereochemical assignments. Recently, we have determined the relative stereochemistry of the C8–C13 hexad by synthesizing the C1–C13 fragment. Presently, we report the assignment of the C17–C25 δ-lactone fragment. NMR analysis of authentic hemicalide along with a computational conformation study allowed us to reduce the number of putative relative isomers from 16 to 4. Concise syntheses of the four candidate diastereomers were achieved using a common strategy based on a Dias aldehyde allylation reaction, an intramolecular Horner–Wadsworth–Emmons olefination, and a dihydroxylation reaction. Finally, thorough NMR comparisons enabled us to deduce the relative stereochemistry of the C1–C17 fragment with high certainty.